Focus on Swap used rather than free RAM
Don't stress about the amount of free (unused) RAM you have. Mac OS X, and other Unix/Linux-based operating systems - and Windows to an extent - tend to use as much free RAM as is available for file caches in order to speed up the performance of running applications, or to keep state of recently closed apps in memory (in case you decide to open them again).
Free RAM is essentially "wasted", because it's its not helping your system run. A lot of people fail to understand this, thinking it somehow akin to "money in the bank". Mac OS X will automatically decrease file caches of infrequently used apps if you open more apps that need the memory.
The thing you need to look at is whether you are using significant (> 100MB) of Swap space. This is application data that has been copied out of RAM (very fast) to your local storage (relatively slow for SSD, extremely slow for HDD!). Mac OS X compressed memory is a sort of waiting room for resources that would otherwise be paged out to disk. I don't worry about a couple of GB of compressed memory, but I do try to minimise the use of Swap space - this is what really kills performance.
HTH,
John
